1. Field of the Invention
The present invention generally relates to methods for avoiding DC current magnetization in magnetic cores, and in particular to use of lasers in the manufacture of magnetic cores so as to break DC magnetizing paths in the cores.
2. Background Description
In the design of magnetic devices, both the alternating and direct current affect the characteristics of the device. When discussing magnetic materials, they are referred to as being "hard" or "soft". Hard materials retain their magnetism after a magnetic field has been applied; e.g. steel. These materials' molecular particles line up their magnetic poles to create magnetic lines of flux, and they stay lined up thereby creating a permanent magnet. Soft materials will magnetize, but will not retain the magnetic properties: their molecular particles will scramble when the magnetic field is removed, leaving the material in its original, non-magnetized state.
Transformers need to be made with soft materials: the magnetic particles must be able to switch direction quickly to concentrate the winding field. A magnetized core will inhibit the transformer action. In practical transformer circuits, it is sometimes necessary to allow a direct current (DC) to flow through a winding. This DC component magnetizes the core and adversely affects the properties of the transformer. To counteract the effects of the direct current, the core must be "gapped", i.e. physically broken so that it does not provide a magnetic path in a complete, continuous circle. This gap breaks the magnetic flux inside the core so as to keep the core material from aligning with the direct current, freeing the material to align with the alternating current, i.e. the signal which is to be operated on by the transformer. The amount of gap is dependent upon the operating characteristics required by the transformer. Among other parameters, the inductance of the core is indirectly proportional to the width of the gap. Direct current capacity (i.e. the ability to handle direct current in the windings without saturating the core) is directly proportional to the width of the gap. High frequency circuits usually need lower inductance, so gapped cores find many uses, such as in switch mode power supplies which have higher direct current. These parameters must be addressed in the final circuit design.
In the prior art, the core gapping process would physically break the core (e.g. as shown at junction 21 in FIG. 2A) into pieces 22 as shown in FIG. 2A and re-assemble the pieces 22 with gapping paper or other non-magnetic material 23 as shown in FIG. 2B, using an adhesive. Cores gapped using this method require fixturing to hold the core in position while the adhesive cures. Cores may also be sliced (with a saw blade) on one side, and filled with an adhesive material. These methods are slow and time consuming. Further, when a manufacturing process involves multiple cores it is necessary to assure that the pieces match when reassembled. Also, the core material is brittle and may crack or chip at undesired locations, producing extra gaps and rendering the core useless. The core surface must be maintained to allow the winding of wire over the core without interference. When gapping cores less than 0.5 inches in diameter, this process becomes difficult due to the small size of the core, which makes it difficult to hold the core in position during the curing phase.